The present invention relates to a method for recycling plastic products or plastic waste articles with painted surfaces.
In recent years, plastic products are often used for automobile parts. For example, when plastic bumpers with unpainted surfaces are recycled, defectively molded bumpers and the bumpers recovered from scrapped vehicles are coarse-crushed to a size about 3 mm cube, which are then mixed with a virgin or new material and reused to mold bumpers. This is possible because the physical properties of recycled plastic materials are not substantially degraded.
However, in the bumpers with painted surfaces, plastic materials recovered in the same manner can not be reused for bumpers. This is because, when the painted bumpers are crushed, paint films are mixed in the recycled plastic materials, so that the physical properties of the plastic materials, such as impact resistance, elongation and surface quality substantially degrade, resulting in a failure to satisfy the performance required as a bumper.
Therefore, the painted plastic bumpers which are defectively molded or those recovered from scrapped vehicles are buried in the ground or burned to thereby cause environmental disruption. Some, however, are recycled for construction piles, in which requirements regarding physical properties are not so strict.
Japanese Patent Publication (KOKAI) No. 3-23909 proposes a method for separating and recovering polyurethane and polyvinyl chloride from waste in which these compounds are laminated. In this method, the above waste composite materials are coarse-crushed to 20 to 50 mm masses, and are then pulverized by agitation in an agitator. In order to recover polyvinyl chloride, polyurethane with a particle size of 1 to 2 mm is separated by a sieve from sheet-like polyvinyl chloride with an overall size of 30 mm.
Currently, for recycling the plastic bumpers with painted surfaces, techniques are being developed to separate the plastic portions from the paint film portions and to recover only the plastic portions for recycling. The following three major techniques are generally used:
(1) Method for removing the paint film by blasting PA1 (2) Method for melting resin and separating the paint film with a screen PA1 (3) Method using chemicals to release the paint film from plastic
Abrasive materials are blown with compressed air against the paint film of the painted plastic bumper to mechanically remove the film.
A painted plastic bumper is coarse-crushed and fed into an extruder where the plastic part is melted. Since the paint film does not melt in the extruder due to its thermosetting property, the paint is separated by a screen installed at the outlet of the extruder.
A mixed liquid of organic salts, polyhydric alcohol and water is heated to about 100.degree. C. A coarse-crushed painted plastic bumper is immersed in this liquid for several hours to release the paint film.
Also, Japanese Patent Publication (KOKAI) No. 2-273207 discloses a method for using vegetable type soft abrasive materials to release paint films by blasting.
Further, Japanese Utility Model Publication (KOKAI) No. 61-196021 discloses a foreign matter removing device for plastic molding material, wherein a foreign matter removal part formed of a wire gauze and a supporting body is serially located between an extruder and a mold.
With the above method (1), it is very difficult to adjust the distance between an abrasive spraying nozzle and an object, the spraying angle, air pressure, and the diameter and shape of the nozzle, because the bumper has a three-dimensional variable surface shape. If the device is not properly adjusted, the paint film is not effectively removed.
In addition, when the plastic parts are recycled, the abrasive materials remain on the plastic surface from which the paint film has been released as if cutting into the surface, so that the physical properties of the recycled material are degraded.
With the method of (2), if a screen with fine mesh is installed at the outlet of the extruder to completely remove the crushed paint film pieces, the extruding pressure must be high, so that the screen is damaged. If a screen with coarse mesh is used, the fine film pieces can not be removed.
The method of (3) can release the finished paint films but can not remove primer without degrading the physical properties of the plastic part. It is also difficult to use the method in industrial applications due to waste disposal problems.
In the method of pulverizing the materials by the agitator rotating at high speed as described in the above Japanese Publication No. 3-23909, it is possible to separate the laminated materials, but the paint films can not be removed from the plastic materials.
Accordingly, it is an object of the invention to provide a method for recycling painted plastic materials, which can omit an expensive step of releasing and separating a paint film by dissolving and uniformly spreading the paint film in the plastic material without adversely affecting the physical properties or surface quality of the plastic materials.